Water flooding (injection of water) is one of the methods in vogue for the secondary recovery of oil when the natural pressure of the reservoir gets diminished. The natural forces responsible for reservoir pressure are: i) expanding gas, ii) encroaching edge water (or bottom water), iii) expansion of oil plus dissolved gas, iv) gravity etc. As soon as a reservoir is struck and produced by a well disturbing its pressure equilibrium, the above forces begin to drain the oil and gas towards the well. During the lifetime of a field and in the course of production, the reservoir pressure and temperature are reduced. Eventually no more economically viable oil production is achieved. Under such situation if the oil production is to sustain, invariably pressure-building operations are to be resorted to in the field. Such operations include gas injection, water flooding or application of polymers etc. These secondary recoveries of oil operations are extremely important and useful for oil production. In addition to this, the secondary recovery of oil in general, is economical and involves less risk than exploration of a new oil field.
Out of the existing, methods, for secondary recovery of oil, water flooding is the most commonly applied and found to be more advantageous. This method promises a high percentage of oil recovery from the reservoir. Large quantities of water have to be injected at a relatively higher pressure into the injection wells.
At times, during secondary recovery of oil operations with water injection, increased water production may result from the oil wells. The increased water cut may be attributed to the Directional Fracture Permeability or Premature (Early) Breakthrough or the mobile waters within the oil field. Interference and/or hydrochemical studies may not be conclusive to prove the contention of Premature/Early Breakthrough. Despite this, the chemical and artificially produced radioactive tracers used for such studies pose handling and hazardous problems. Hence, such cases warrant development of techniques making use of environmental isotopes present in the injection water to identify the movement of injection water and establish Premature (Early) Breakthrough phenomenon in oil wells.
The method utilizing natural carbon-13 isotope present in the Injection Water for identification of early breakthrough phenomenon was carried out in one of the Oil Fields in India, where injection has been carried out by the oilfield personnel using groundwater. Since the method makes use of carbon-13 isotope present in the injection water, it avoids the handling problems associated with the addition of foreign substances into the injection water stream. Preliminary studies have indicated that the isotope contents of injected water and formation waters are different having a bearing on the depth of their occurrence. In case of environmental tracers, such as radioisotopes (tritium and radiocarbon) and stable isotopes (deuterium, oxygen-18 and carbon-13), because of their possible precision measurements using mass spectrometers even at 10−12 to 10−18 isotope ratio levels, were envisaged to be studied in the selected oil field. Preliminary isotope measurements on these samples revealed that the tritium content of the injected water was negligible (<5 T.U detection limit of our measuring system) and its further decay in the reservoir would make its detection still more formidable. The radiocarbon isotope (14C) concentration was to be high in the groundwater as compared with the normal formation water. Similarly δ13C concentration of the injection water showed contrast with respect to the formation/reservoir water. Thus the development of this method took place.
The carbon-13 isotope concentration of natural waters is measured against the most abundant carbon-12 isotope, and the ratio (13C/12C) is expressed as per milli (×1000) deviation (δ(delta) ‰) from the Pee Dee Belemnite Carbonate value:δ13C‰=((13C/12Csample−13C/12Cstandard)/(13C/12Cstandard))×1000
Carbon isotopic composition of dissolved carbonate species in many aquifers have proved to be a valuable tool in defining the source and the history of these species. There is not much literature on δ13C values of inorganic carbon as far as oil field waters are concerned. However δ13C values of CO2 and carbonate cement of petroleum reservoir are commonly reported. The average δ13C value of present atmospheric CO2 is −7.0‰. On the other hand, the deep formation waters which have equilibrated with lime stone would have δ13C in the range 0 to +3.0‰ or even much positive values can be possible because of loss of CO2 from these waters. The concentration of δ13C can also be complicated by several non-mineral sources such as introduction of heavy CO2 by thermal solutions or relative light CO2 through oxidation, dissolved oxygen or sulphate reduction of carbon bearing substances.
However, in our method development, these are obviated as we not only studied spatial but also temporal variations in δ13C of producing oil well waters as a result of water injection. Hence the present method development is essentially based on the variation of δ13C concentration in the injection and formation waters.
The technique of using 87Sr isotope for injection water study (U.S. Pat. No. 4,743,761) was based on the ratio of radioactive 87Sr to that of stable 86Sr isotope. For comparative study the Strontium isotopic compositions have to be measured at 0.01 to 0.02% relative standard deviation. For the measurement of 87Sr/86Sr ratio, more sophisticated and costlier instrument (Thermal Ionization Mass Spectrometer; TIMS) is needed. The Sr isotope measurements entail cumbersome and time consuming sample preparation procedures, which result in very low throughput. Sr can get precipitated as salts, and storage of brines and retrieval of pristine Sr isotopic/elemental abundances could be difficult, thus require greater care in storage.
In the case of chemical compounds (U.S. Pat. No. 3,851,171), the process involves artificial addition of water-soluble substituted stilbene compound to the injection water prior to its injection, and the produced water is analyzed for stilbene compound in it. Thus the chemical tracers involve additional expenditure and handling problems.
Thus we have developed a method, to obviate above drawbacks, for identification of Early Breakthrough phenomenon of injection water and its movement in oil wells utilizing natural carbon-13 stable isotope as a tracer, and the advantage of using carbon-13 stable isotope lies in the fact that it is present in the injection water as well as in the formation water in varying proportion. Water sample handling for δ13C measurement is relatively easy and requires relatively less sophisticated equipment. δ13C values cover a wide range +10 to −25‰ for inorganic carbon.